Hematopoietic stem cells are the source of all hematopoietic cells, and replenish the hematopoietic compartment as required throughout organism lifespan. Since alterations in the equilibrium of this compartment greatly impact stem cell maintenance, the molecular mechanisms regulating the cell fate decisions of stem cells hold great promise for clinical applications. Studies of genetically-engineered mouse models suggest that metabolic cues contribute to the governance of these cells' self-renewal capacity. To date, however, little is known regarding the role of lipid metabolism in stem cell homeostasis. To better understand the key metabolic pathways involved in stem cell fate and maintenance, we propose the following Specific Aims: 1. To investigate the effects of inactivation of PPAR-fatty acid oxidation in stem cells; We have previously shown that stem cells exhibit higher Ppard expression and fatty acid oxidation than committed progenitor cells, and have hypothesized that lipid metabolism plays a role in their repopulation capacity. In accordance with this premise, we have also found that inhibition of fatty acid oxidation in vitro leads to a reduction of long-term culture-initiating cell capacity. Furthermore, Ppard-ablation leads to reduction of fatty acid oxidation in the hematopoietic stem cell compartment. The current proposal aims to elucidate the effect of genetic loss of Ppard in vivo on the reconstitution ability of stem cells in a transplantation setting. Stem cell division assays with Ppard knockout models will allow us to test whether Ppard-ablation leads to increased commitment of stem cells during their division. 2. To enable long-term engraftment with minimal donor cells by the activation of PPAR signaling; we will employ different activators of PPAR? at low doses in vivo to observe their effects on the long-term maintenance of murine stem cells from Ppard wild-type and knockout mice. This will provide a definitive proof, in a PPAR?-dependent manner, of the potential benefit of PPAR? activators in the stem cell compartment. We will then determine, through the use of xenograft mouse models transplanted with human bone marrow cells, whether pharmacological activation of PPAR signaling induces a transplanted minimum number of human hematopoietic stem cells to maximize their long-term repopulation capacity in vivo. 3. To identify cell fate determinants that maintain stem cell-ness through division balance control; in stark contrast to what is known about symmetric and asymmetric division of normal cells in invertebrates, it has been extremely difficult to image the division pattern of most purified stem cell compartments in vertebrates. We therefore propose to generate knock-in mouse lines for real-time imaging of stem cell divisions and to study the intrinsic and extrinsic signals regulating stem cell decision. Combined with the data from our whole transcriptome analysis by RNA-seq in the purified stem cell compartment, the results from these mouse models will lead to a deeper understanding of the cell fate determinants of stem cells. These proposed studies will identify a novel metabolic switch for the cell fate decisions of stem cells, and in turn open new therapeutic avenues for the manipulation of hematopoietic stem cell function, and possibly the function of leukemia stem cells. This work will be conducted with the support of the following experts; Drs. Michael A. Brownlee (Metabolism), Chih-Hao Lee (Metabolism), David E. Avigan (Hematology/Oncology), Julie Teruya-Feldstein (Hemato- pathology), Toshio Suda (Stem Cells), Jan Vijg (Genetics), and Winfried Edelman (Gene Targeting). Importantly, Dr. Paul S. Frenette (Stem Cell niche) is closely supporting our research program along with Dr. Arthur Skoultchi (Hematology).